1. Field of the Invention
The present invention relates to a process for preparing saturated carboxylic acids having from 1 to 4 carbon atoms and to an apparatus for carrying out this process.
2. The Prior Art
It is known that acetic acid can be prepared by gas-phase oxidation of C4-hydrocarbons in the presence of a catalyst. Most prior art processes provide for the reaction gas mixture to be passed over the catalyst once, to separate off the resulting acetic acid by condensation or scrubbing and to discard the remaining gas. For example, U.S. Pat. No. 3,917,682 describes a procedure in which the acetic acid is obtained by oxidation of butene in the presence of a Ti/V catalyst having a high proportion of rutile. Here the acetic acid is isolated by partial condensation of the reaction mixture and the remainder of the reaction gas is not recirculated. Such processes have to achieve a high butene conversion on a single pass through the reactor, which can be achieved only at low yields or low space-time throughputs. For this reason, an economically satisfactory process has not yet been developed on the basis of this process concept.
It is known from U.S. Pat. No. 4,146,734 that the gas-phase oxidation of butene to acetic acid can be carried out in the presence of a catalyst comprising lanthanide compounds. A method of isolating the acetic acid and further desired compounds formed during the gas-phase oxidation is not indicated.
DE-A 2,149,752 and DE-A 1,279,011 describe processes for the catalytic gas-phase oxidation of butene to acetic acid in the presence of specific catalysts. A disadvantage of these processes is that the formic acid formed as desirable compound decomposes during the recirculation of the noncondensable part of the reaction gas.
DE-A 1,921,503 refers to the possibility of, in the preparation of acetic acid by catalytic gas-phase oxidation of butene, recirculating the unreacted part of the reaction mixture to the reactor. However, express reference is made to the uneconomical nature of a circulating gas process.
The process was developed to the pilot plant scale by Chemische Werke Hxc3xcls and is described in various publication (R. P. Lowry, A. Aguilo, Hydrocarbon Processing, 10, (1974), 103; PEP Report No. 37A (1973)). It provides for the direct, untreated recirculation of ⅘ of the gas mixture leaving the reactor (FIG. 1). In this embodiment, the reaction product is partly circulated without the acids being separated off and only part is taken off for isolation of acetic acid. In this process, there is significant accumulation of organic acids in the reaction gas, as a result of which both acetic acid and formic acid are obtained only in unsatisfactory yield.
It is an object of the present invention to provide a process for preparing saturated carboxylic acids having from 1 to 4 carbon atoms, in particular acetic acid, by gas-phase oxidation of saturated and/or unsaturated C4-hydrocarbons, which gives high acid yields and in which the by-products are obtained as useful materials.
It has surprisingly been found that the preparation of saturated carboxylic acids having from 1 to 4 carbon atoms by gas-phase oxidation of saturated and/or unsaturated C4-hydrocarbons can be carried out with particularly high yields. These high yields will result if, in contrast to the above-mentioned prior art processes, a substream which has been substantially freed of acids, of the gas mixture leaving the reactor, is recirculated to the reactor inlet.
The present invention provides a process for preparing saturated carboxylic acids having from 1 to 4 carbon atoms by gas-phase oxidation at a reaction temperature of from 100xc2x0 C. to 400xc2x0 C. and pressures of from 1.2xc3x97105 Pa to 51xc3x97105 Pa. This oxidation occurs by the reaction of saturated and/or unsaturated C4-hydrocarbons, with an oxygen-containing gas and water vapor in the presence of at least one catalyst. The gas leaving the reactor is partly recirculated in a reaction gas circuit. This reaction gas circuit is configured such that part of the organic acids formed in the gas-phase oxidation is taken from the gas leaving the reactor, so that the acid content of the recirculated part of the gas leaving the reactor is from 0.01% to 6.0% by volume.
The saturated or unsaturated hydrocarbons having 4 carbon atoms are compounds selected from the group consisting of n-butane, i-butane, 1-butene, cis-2-butene, trans-2-butene, isobutene and 1,3-butadiene. Preference is given to n-butane and the butene isomers 1-butene, trans-2-butene and cis-2-butene and also mixtures comprising high proportions of these compounds. In the process of the invention, the C4-hydrocarbon fraction can further comprise linear and/or branched and/or cyclic hydrocarbons having more or less than 4 carbon atoms, for example methane, ethane, ethene, propene, propane, pentanes, pentenes, pentadienes, cyclopentane, cyclopentene, cyclopentadiene and methylcyclopentane. Likewise, alcohols, aldehydes, ethers, ketones and esters having from 1 to 8 carbon atoms may be present. Preferred starting materials are cheap feedstock mixtures from petrochemical processing, e.g. xe2x80x9cC4 fractionxe2x80x9d (predominantly butadiene and i-butene), xe2x80x9craffinate 1xe2x80x9d (predominantly i-butene and n-butenes) and xe2x80x9craffinate 2xe2x80x9d (predominantly butanes, 1-butene and 2-butenes) or mixtures comprising such hydrocarbons. These can, if desired, be used after a pretreatment, e.g. a purification or hydrogenation.
The reaction temperature in the gas-phase oxidation is generally from 100xc2x0 C. to 400xc2x0 C., preferably from 150xc2x0 C. to 250xc2x0 C., particularly preferably from 180xc2x0 C. to 230xc2x0 C. The reaction is generally carried out at pressures of from 1.2xc3x97105 Pa to 51xc3x97105 Pa, preferably from 4xc3x97105 Pa and 31xc3x97105 Pa, particularly preferably from 9xc3x97105 and 17xc3x97105 Pa.
As oxygen-containing gas, it is possible to use air, air enriched with oxygen and preferably pure oxygen. An inert gas such as nitrogen can also be present in the process of the invention.
The proportion by volume of water vapor in the reactor inlet gas consisting of water vapor, oxygen-containing gas, C4-hydrocarbons and inert gases fed to the reactor is generally from 5% to 80% by volume, preferably from 5% to 40% by volume, particularly preferably from 5% to 30% by volume.
The proportion of butene, which may be present as starting material either alone or in admixture with further C4-hydrocarbons, is from 1% to 5% by volume, preferably from 1.5% to 3% by volume. The proportion of butane, which likewise can be present as starting material either alone or in admixture with further C4-hydrocarbons, is from 5% to 80% by volume, preferably from 5% to 60% by volume, particularly preferably from 10% to 50% by volume.
The oxygen content of the reactor inlet gas is from 1% to 35% by volume, preferably from 2% to 20% by volume, particularly preferably from 3% to 12% by volume.
In another embodiment, a proportion of inert gas of from 0% to 25% by volume can be fed in. The proportion of carbon oxides and further reaction by-products in the reactor inlet gas depends on the reaction procedure and the separation of acids. This is generally from 10% to 80% by volume, preferably from 15% to 65% by volume. The percentages by volume of the individual constituents of the reactor inlet gas in each case add up to 100% by volume.
Suitable catalysts for the process of the invention are all catalysts which have been generally described for the partial oxidation of saturated and/or unsaturated C4-hydrocarbons to acetic acid. Preference is given to mixed oxide catalysts comprising the vanadium oxides; and particular preference is given to coated catalysts as are described in DE-A 19,649,426. The disclosure of DE-A 1 9,649,426 is herewith incorporated by reference into the present application. This catalyst is a coated catalyst comprising an inert nonporous support body and a catalytically active amount of a mixed oxide composition applied to the outer surface of the support body. The catalytically active composition comprises (a) one or more oxides selected from the group consisting of titanium dioxide, zirconium dioxide, tin dioxide and aluminum oxide and (b) from 0.1% to 1.5% by weight, based on the weight of the component (a) and per m2/g of specific surface area of the component (a), of vanadium pentoxide.
As additional component (a), it is possible for one or more oxides selected from the group consisting of oxides of boron, silicon, hafnium, niobium, tungsten, lanthanum and cerium to be present. If the component (a) is doped with the oxides specified, they are generally present in an amount of from 1% to 30% by weight, based on the total weight of the component (a).
In the component (b), it is possible, if desired, for part of the vanadium pentoxide, preferably from 10% to 90% by weight, to be replaced by one or more oxides of molybdenum, chromium and antimony. If desired, one or more oxides of alkali metals, elements of main groups V and VI of the Periodic Table of the Elements and the transition metals may also be present as additional component (b). In general, the amount of these dopants is from 0.005% to 15% by weight, calculated as oxides and based on the total weight of the component (b).
Preference is given to compositions having a high surface area of the component (a) of from 40 to 300 m2/g, with tin oxide, niobium oxide or tungsten oxide being able to be present if desired, and having a component (b) which is doped with Mo and/or Cr and/or Sb and/or Au. The catalytically active mixed oxide composition may, if desired, also contain from 10% to 50% by weight, based on the total weight of the catalytically active mixed oxide composition, of inert diluents such as silicon dioxide, silicon carbide and graphite.
The catalytically active mixed oxide composition is applied as a shell to the outer surface of the support body in an amount of from 1% to 40% by weight, preferably from 5% to 25% by weight, in each case based on the total weight of support body and active composition. The thickness of the layer is generally from 10 to 2000 xcexcm, preferably from 100 to 1000 xcexcm. The coated catalyst may also comprise a plurality of layers having different compositions. It is also possible for one or more constituents of the active components (a) and (b) to be present in different concentrations in the individual layers.
Suitable materials for the inert, nonporous support body are all nonporous materials which are inert under the operating conditions of the gas-phase oxidation and are stable over the operating period. Examples are steatite, Duranit, silicon carbide, magnesium oxide, silicon oxide, silicates, aluminates, metals such as stainless steel and also, if desired, mixtures of these materials. Preference is given to ceramic materials such as steatite. The inert, nonporous support body can have any desired shape. Examples of suitable shapes are spheres, cylinders, cuboids, tori, saddles, spindles and helices. Likewise suitable as support are ordered packings such as monoliths or cross-channel structures. Preference is given to support shapes having as high as possible a geometric surface area per unit volume, for example rings.
The dimensions of the support bodies are determined by the reactors for the gas-phase oxidation. In general, the shaped bodies have a length or a diameter of from 2 to 20 mm. The wall thickness, for example in the case of rings or hollow cylinders, is advantageously from 0.1 to 4 mm.
In the process of the invention, the reaction gas circuit is configured such that part of the organic acids, primarily formic acid and acetic acid, present in the gas leaving the reactor is taken from this gas so that the partial pressure of these acids at the reactor inlet remains low. In general, the proportion of acid is reduced to from 0.01% to 6% by volume, preferably from 0.1% to 3% by volume, particularly preferably from 0.2% to 2% by volume. Unreacted C4-hydrocarbons and intermediates such as acetaldehyde, acetone, methyl ethyl ketone and 2-butanol which can be reacted further to form acetic acid mostly remain in the circulated gas and are returned to the reactor inlet.
The process of the invention can be used to reduce the acid content to the abovementioned residual acid content in part of the gas leaving the reactor, generally from 60% to 99.8% by weight, preferably from 90% to 99.5% by weight. Subsequently, this part of the reactor outlet gas is recirculated to the reactor. The untreated part of the reactor outlet gas is then discarded and can, for example, be flared off. The proportion of untreated reactor outlet gas depends on the amount of carbon oxides COx formed because these have to be discharged via this branch stream. They can then be disposed of by incineration.
An alternative embodiment of the process of the invention is to reduce the acid content of the reactor outlet gas to the abovementioned residual content immediately after the gas has left the reactor. Then the treated reactor outlet gas is recirculated to the reactor either completely or partially. It is preferably recirculated to an extent of from 60% to 99.8% by weight, particularly preferably from 90% to 99.5% by weight. This embodiment is particularly preferred since the target products, viz. the carboxylic acids, are substantially separated off beforehand and are not incinerated.
The organic acids can be separated off by known methods or a combination of these methods. Examples of suitable methods are partial condensation of the gas mixture; rectification, with or without addition of auxiliaries (e.g. extractive rectification); absorption of the acids in a suitable solvent; separation by means of a membrane and absorption by a solid absorbent. Preference is given to separating off the organic acids by means of partial condensation of the gas mixture, for example in a condenser, and subsequently dividing the remaining gas mixture.
The mass flow of recirculated gas is generally from 1 to 100 times, preferably from 5 to 80 times, particularly preferably from 10 to 40 times, the mass flow of fresh starting materials fed in.
The crude acid which has been separated off is dewatered and purified using suitable customary methods either alone or in combination, e.g. liquidxe2x80x94liquid extraction, extractive rectification, azeotropic rectification and rectification. The process of the invention is preferably used for preparing acetic acid and formic acid, particularly preferably acetic acid. An important advantage of the process of the invention is that in the preparation of acetic acid the by-products formed are obtained as useful materials, especially in the form of formic acid. In contrast, in the processes known from the prior art the formic acid formed as an intermediate is decomposed to form COX compounds which have to be disposed of by incineration.
As the reactor, it is possible to use apparatus embodiments which are suitable for carrying out oxidation reactions in the gas phase and are able to remove the high heat of reaction without excessive heating of the reaction mixture. The process of the invention can be carried out continuously or intermittently, i.e. a constant reactor inlet mixture can be fed in or the feed composition can vary cyclically. The gas mixture can react over a catalyst in a fixed bed, for example in a multitube reactor or tray reactor, or in a moving or fluidized bed. Preference is given to cooled multitube reactors containing a fixed catalyst bed. Particular preference is given to embodiments in which the individual tubes making up the multitube reactor have an internal diameter of from 10 mm to 50 mm and a length of from 1 m to 6 m. The flow velocity (based on the empty tube) in the reaction tubes is generally from 0.1 m/s to 10 m/s, preferably from 0.3 m/s to 5 m/s, particularly preferably from 0.5 to 3 m/s.
The reaction tubes can be charged with catalysts having different compositions, shapes and dimensions. The charge can have been introduced into the reaction tubes so as to be homogeneous or vary zonewise in the axial direction. Each zone can contain randomly diluted or mixed catalysts.